1. Field of the Invention
The present invention relates to a management technique of a friction coefficient based on surface roughness, an information recording medium substrate, an information recording medium and a manufacture method thereof.
2. Description of the Related Art
There is a magnetic recording medium loaded with HDD as an information recording medium. A remarkably rapid increase of a recording capacity of a hard disk drive (HDD) in recent years is realized, for one thing, by lowering (lowering glide) a gap between a magnetic head (a head) and a magnetic recording medium (a medium) during recording/reproduction (head flying height). The lowering of the gap between the head and the medium is realized by an attempt to smooth a medium surface, but the smoothed medium surface causes an adsorption problem between the head and the medium. Therefore, design of the medium surface is always troubled with trade-off between the lowering of glide and avoiding of head adsorption.
In order to solve mutually contradictory problems of the smoothing of the medium surface required for the lowered glide, and the avoiding of an adsorption tendency accompanied by the smoothing (=tendency of increase of a friction coefficient), a precision surface design is necessary.
For shape management of the medium surface, from a point of view that process feedback is easy, surface roughness parameters such as Rmax and Ra by an atomic force microscope (AFM), normalized roughness Ra/Rmax, and the like have heretofore been used.
Herein, Ra and Rmax are defined by the Japanese Industrial Standard (JIS B0601). Rmax is the above-mentioned maximum height (the distance from a highest peak to a lowest valley), Ra is the above-mentioned center-line-mean roughness (the average of an absolute value of a deviation from a center line of a roughness curve to the roughness curve.
However, by checking a relation between Rmax, Ra or Ra/Rmax measured by AFM and the friction coefficient, it has been found that in a low glide area (about 10 nm or less) requiring more precise surface design, such surface management technique is extremely bad in sensitivity. FIGS. 1 and 2 show the relation between the substrate surface roughness (Rmax (FIG. 1), Ra (FIG. 2)) measured by AFM and the friction coefficient. Even in the same Rmax (e.g. Rmax of about 7.5 nm) and the friction coefficient with a range of 0.7 to 2.2, and it is thus impossible to manage the friction coefficient with the parameters such as Rmax and Ra.
On the other hand, there is proposed a technique of taking a correlation between a bearing area (bearing ratio) and the friction coefficient to manage the friction coefficient. Specifically, in a magnetic recording medium constituted by forming texture on a medium surface, there is proposed a technique of forming the texture (concave/convex) in such a manner that the bearing area in a depth of 20 nm from a surface top portion is 20% or less, and defining the friction coefficient to be small (Japanese Patent Application Laid-Open No. 189756/1993). Additionally, the bearing area means a proportion occupied by an area appearing when the concave/convex in a measured area is cut on an arbitrary equal height surface (horizontal surface) in the measured area, and can be measured using the atomic force microscope (AFM) or the like.
However, the technique principally aims at the magnetic recording medium formed by polishing an aluminum alloy substrate surface with an NiP film formed thereon with a free abrasive grain and performing a texture processing, and uses the bearing area in the depth of 20 nm from the surface top portion as the parameter (i.e., a rough magnetic recording medium with a surface roughness of 20 nm or more is an object). Therefore, there is a problem that with respect to the magnetic recording medium having a surface roughness of Rmax 15 nm or less, the technique is completely useless as a friction-coefficient management technique.
Additionally, the technique is derived from an experiment, and is not derived based on theories such as a real contact area described later.
Furthermore, for example, when a texture forming method differs, the total number of protrusions, protrusion mode (protrusion curvature radius or horizontal sectional shape, protrusion height), and the like differ. Even with the same Rmax or Ra, the friction coefficient by the medium surface differs. Therefore, in this case, the aforementioned technique is completely useless as the management technique of the friction coefficient of the medium surface.